1. Field of the Invention
The present invention relates to a system for purifying exhaust gas containing nitrogen oxides (NOx), which is emitted from an internal combustion engine from a mobile body, particularly from a vehicle.
2. Description of the Related Art
In order to purify carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and the like, which are contained in exhaust gas emitted from an internal combustion engine of a mobile body such a vehicle, a three-way catalyst functioning under a stoichiometric air-fuel ratio and an exhaust gas purifying system using the same have been used.
Meanwhile, a lean burn engine causing combustion in an air-fuel ratio range higher than the stoichiometric air-fuel ratio for improving fuel efficiency has been put into practical use. As a device for purifying emitted nitrogen oxides (NOx) under a lean burn condition, a NOx treating catalyst having a function of trapping NOx has been used. The NOx treating catalyst for lean burn includes a NOx trap agent (NOx adsorbent) as well as a catalyst for reducing and treating NOx. The NOx treating catalyst traps NOx in the trap agent when the air-fuel ratio is lean (i. e. a fuel ratio is low), and reduces the trapped NOx with reducing gas components in the exhaust gas when the air-fuel ratio is rich (i. e. a fuel ratio is high). Specifically, in this NOx treating catalyst, when the air-fuel ratio is rich, the reducing gas components such as hydrocarbons (HC), carbon monoxide (CO) and hydrogen (H2), which are emitted from the internal combustion engine, are supplied to the NOx reducing/treating catalyst, and NOx is treated by a reduction reaction to be discharged as N2. Note that a reduction function of each reduction gas depends on a temperature condition. In order to allow HC and CO to exert an excellent reduction function in a treatment reaction of NOx, a temperature condition of 250xc2x0 C. or higher is required.
However, in general, an exhaust gas temperature under the lean burn condition is low, and it is difficult for the exhaust gas temperature to reach the temperature of 250xc2x0 C. or higher where HC and CO exert the excellent reduction function for treating NOx. Particularly, since an exhaust gas temperature at the time of engine start, which is called a cold range, is a temperature ranging from 180 to 200xc2x0 C., the reduction/treatment of NOx by means of the NOx treating catalyst cannot be performed efficiently.
Moreover, in the case of a vehicle using a diesel engine, since the exhaust gas temperature is further lowered, reduction/treatment efficiency of the NOx treating catalyst is deteriorated more.
Meanwhile, objects of exhaust gas control for a vehicle include not only NOx but also HC gas. As described above, when the exhaust gas temperature is low and HC and CO in the exhaust gas are not used for the reduction reaction in the NOx treating catalyst, HC and CO are not consumed by the NOx treating catalyst and flow out to the outside as exhaust gas.
Note that, for a problem about flowing out of HC at a low temperature in the lean burn internal combustion engine, the Japanese Patent Laid-Open publication No. 2000-257417 discloses an exhaust purifying apparatus, in which a HC trap agent for trapping HC of a high carbon number and an oxidation catalyst for treating HC of a low carbon number are disposed downstream of the NOx treating catalyst.
However, in the above-described exhaust purifying apparatus, NOx treatment is not performed at a low temperature but performed by raising a temperature. By such a raised temperature, HC trapped in the downstream HC trap agent is also treated. Specifically, in this apparatus, since a heating device is always used, an increase in fuel consumption is brought. Therefore, good fuel efficiency that is the important advantage given by lean burn cannot be obtained.
An object of the present invention is to provide a purifying system for a vehicle, which is capable of treating NOx and HC in exhaust gas more efficiently under the lean burn condition.
In order to achieve the object, an exhaust gas purifying system for a vehicle according to an aspect of the present invention includes a hydrogen enriching device that increases component ratio of hydrogen with respect to reducing gas components in exhaust gas, the hydrogen enriching device being disposed in an exhaust gas passageway, a NOx treating catalyst disposed downstream of the hydrogen enriching device in the exhaust gas passageway, and a HC trap catalyst disposed downstream of the NOx treating catalyst in the exhaust gas passageway. Note that, the exhaust gas flows from the hydrogen enriching device direct to the NOx treating catalyst continuously during operation.